1. Field of the Invention
The present invention relates to a nitride semiconductor light emitting diode formed on a substrate made of a nitride semiconductor (AlxInyGa1-x-yN: 0≦x≦1, 0≦y≦1, 0≦x+y≦1).
2. Description of the Related Art
Nitride semiconductor light emitting elements using a nitride semiconductor, such as a laser diode and a light emitting diode enables high luminance light emission within a wide wavelength range from an ultraviolet range to a visible range, and have widely been employed in a light source for optical disk, a light source for backlight, a light source for illumination and the like.
Since a substrate made of a nitride semiconductor has hitherto been hardly available, the nitride semiconductor light emitting element has been usually manufactured by growing a nitride semiconductor layer on a different type of a substrate made of sapphire or the like. However, when the nitride semiconductor layer is grown using a different type of the substrate made of sapphire or the like, many threading dislocations are generated by lattice constant mismatch between the substrate and the nitride semiconductor. Therefore, as compared with the case of using the other compound semiconductor, density of defects such as dislocation in the semiconductor layer remarkably increases. Particularly in case of the laser diode, when the dislocation density in a semiconductor layer in a light emitting region, at which laser oscillation is obtained, is high, element lifetime drastically decreases, resulting in practically serious obstacle.
Therefore, there have been proposed various methods in which a semiconductor light emitting element is manufactured by growing a nitride semiconductor layer that has low density of defects such as dislocation and has satisfactory crystallinity.
For example, JP-A-2000-174392 discloses that a protective film is partially provided on a surface of a gallium nitride-based compound semiconductor layer grown on a sapphire substrate and a nitride semiconductor is selectively grown thereon, thereby remarkably decreasing a dislocation density in the nitride semiconductor layer, and then a second buffer layer grown at 200 to 900° C. is grown, thereby making crystallinity in the surface uniform, and a laser diode made of a nitride semiconductor having satisfactory crystallinity is formed thereon.
JP-A-2002-261014 discloses that a nitride semiconductor is grown in a large film thickness on a sapphire substrate by a halide vapor phase epitaxial method, and then removing the sapphire substrate to manufacture a nitride semiconductor substrate, and a laser diode is formed thereon.
JP-A-2003-327497 discloses that a striped or circular mask is formed on a GaAs substrate and a GaN layer is vapor-phase grown thereon, and then the GaAs substrate is removed to manufacture a GaN single crystal substrate. Next, a surface thereof is polished and heat-treated in a mixed gas atmosphere containing an NH3 gas, and then a nitride semiconductor layer in which generation of dislocation is suppressed is grown on the GaN single crystal substrate to form a laser diode or a light emitting diode.
Furthermore, JP-A-2006-24713 disclosed a semiconductor laser element in which an n-type nitride semiconductor layer 28, an active layer 32 and a p-type nitride semiconductor layer 34 are grown on an n-type GaN substrate 10, as shown in FIG. 11, to form a ridge 34a on the p-type nitride semiconductor layer 34. JP-A-2006-24713 proposes that a threading dislocation density of the entire element is decreased by use of a GaN substrate 10, and a V-groove 100 parallel to a ridge stripe is formed in a semiconductor layer thereby intentionally generating dislocation 42 to provide a threading dislocation concentration region 102 where dislocations are concentrate along the V-groove 100. According to JP-A-2006-24713, when the threading dislocation concentration region 102 is provided in the active layer 32, compressive strain to be applied to the active layer 32 is relaxed and thus generation of defects caused by the compressive strain can be prevented. In this semiconductor laser element, the downward portion of the ridge 32a becomes a light emitting region. However, since the light emitting region is formed away from a threading dislocation concentration region 102, the threading dislocation density of the light emitting region is decreased by employing the GaN substrate 10.
As described above, there have hitherto been proposed various methods in which a nitride semiconductor light emitting element is formed by using a substrate made of a nitride semiconductor such as a GaN substrate. However, any method aimed at obtaining a laser diode having excellent life property by decreasing a dislocation density in a light emitting region of the laser diode. However, it is possible to expect the following advantages that are not achieved by the laser diode: when a surface emitting type light emitting diode is formed by using a substrate made of a nitride semiconductor such as a GaN substrate, not only a nitride semiconductor having satisfactory crystallinity can be employed, but also extraction efficiency of light emitted in the active layer is remarkably improved.
As shown in FIG. 10A, in a light emitting diode including a nitride semiconductor layer 35 grown on a sapphire substrate 8, since a refractive index of a sapphire substrate 8 or an electrode 36 is smaller than that of the nitride semiconductor layer 35, a structure like a wave guide in which top and bottom of the nitride semiconductor layer 35 are interposed between layers having a small refractive index is formed, and thus light emitted in the nitride semiconductor layer 35 is likely to undergo multiple reflection inside the nitride semiconductor layer 35. Light that underwent multiple reflection inside the nitride semiconductor layer 35 undergoes absorption loss due to the nitride semiconductor layer 35 per se or the electrode 36. Moreover, since the nitride semiconductor layer 35 usually has a thickness of several μm, whereas, the element dimension in a plane direction is at least several 100 μm, the number of multiple reflection inside the nitride semiconductor layer 35 drastically increases, and thus external quantum efficiency of the nitride semiconductor light emitting diode considerably decreases.
In contrast, as shown in FIG. 10B, when a light emitting diode is formed by using a substrate 10 made of a nitride semiconductor, such as GaN substrate in place of a different type of a substrate 8 such as a sapphire substrate, the refractive index of the substrate 10 approximately agrees with that of nitride semiconductor layer 35 constituting the element. Therefore, it is possible to overcome multiple reflection inside the thin nitride semiconductor layer 35 and to reduce absorption loss in the nitride semiconductor layer 35 or the electrode 36, resulting in improvement of external quantum efficiency.
Furthermore, when a substrate made of a nitride semiconductor, such as a GaN substrate is used, heat radiation efficiency can be improved because of its higher thermal conductivity than that of the sapphire substrate. Therefore, an improvement in various performances of the light emitting diode can be expected, for example, it becomes possible to control the piezoelectric field using a nonpolar/semipolar plane of the GaN substrate.
However, the present inventors have actually manufactured a light emitting diode having the same semiconductor laminated structure as that of a light emitting diode of a conventional sapphire substrate by using a substrate made of a nitride semiconductor, and found that there is a problem that electro-optical characteristics of the light emitting diode deteriorate as compared with the case of forming on the sapphire substrate, contrary to conventional expectations. For example, when a comparison is made between the light emitting diode formed on the GaN substrate and the light emitting diode formed on the sapphire substrate, although the light emitting diode formed on the GaN substrate has a small dislocation density and the semiconductor layer has satisfactory crystallinity, a forward voltage (hereinafter referred simply to “Vf”) increased by at least about 20%, while the output decreased by at least about 30%. When using the GaN substrate, light extraction efficiency should be increased by about 5% as compared with the case of using the sapphire substrate, and a decrease in output by at least about 30% means that internal quantum efficiency decreased furthermore.